Production of nitrogen-bearing stainless steel



Patented Jan; 9, 1951 PRODUCTION OF NITBQGEN-BEARING Harry- 'lanczyn, Baltimore; Md}, assignorto Armco .Steel- Corporation;

Ohio

a corporation or No Drawing. ApplicatiomMarclt 19,. 1946,,

Serial'No. 65,5;fi8,

4 Claims; (Cl: 754-4 2):

This invention relates to stainless steel, more especially to an art of producing thesame.

An object of my invention is the provision of an efiicient, reliable and thoroughly practical process for producing nitrogen-containing stain-'- less steel, which is readily executed with minimum departure from standard stainless steel metallurgical production operations and equipment i A further, object of my invention is the simple, efiicient and economical production of stainless steels, of desirable physical properties made to a desired specification of'i'ron', carbon, chrog mium and nitrogen, either with orwithout supplementary elements such as nickel,f copper, manganese, cob-alt, columbium, titanium, molybdenum, tantalum, tungsten, vanadium and'the like.

A still further object of my invention is the production of inexpensive austenitic stainless steels wherein substantial quantities. of nitrogen are employed as an austenite-forming ingredient;

Other objects of. the invention in part. will be obvious and in part pointed out hereinafter.v

The invention accordingly consists in the com,- bination of elements, composition of ingredients and" mixture of materials, and in the several steps and the relation of each of the same to; one or more of the, others as.describedihereih,

and the scope of the application of. whichisindicatedin the following claims. As conducive to a clearer understanding of certain features of my invention it may be} noted at this point that the addition of a small percentage of nitrogen to stainless steel, as taught in the prior art, is known to lend many beneficial characteristics to the metal. In the straight chromium grades, the stainless steel containing nitrogen is ofv an inherently fine, even grain structure, this being particularly important in the high chromiumgrades; theyar'e'ofimproved workability over a- Wider range of working temperatures; theyare less subject to grain growth, brittleness, decarburization and. fatigue; and they are more durable and of high impact values than are similar stainless steels which contain only incidental quantities of nitrogen. Inthe chromium-nickel grades, the steels are rendered austenitic with much lower quantities of nickel, nitrogen acting as a substitute for nickel in whole or in part, thus effecting direct savings in this expensive alloying ingredient;

The successful and consistent production of 2 nitrogen-bearing stainless steel, however; has heretofore presented a number of problems. Where nitrogen introduced; directly during the meltin f'proc'ess; as by nitrogengas' or ammonia, theresults are erratic; In some-instances; a desired nit'rogen content is achieved, in others it snot. No means areknownfor successfully and consistently addingdesired amounts of nitrogen te mclt'en stainless steel. Where highnitrogen ferrochrome is employed as a source of nitrogen; the results are less er"- ratic but still leave much to be desired. The addition of nitrogen by this procedure is slow, costly and falls short of the practical degree of reliability needed for the rapid and uniform production of nitrogenized products of desired alloycontents;

One of the outstanding objects of myinvention is the efli'cient; economical and thoroughly practical production of stainless steel, either with or without supplementary additions indicated above; whereinadesired nitrogen content is rapidly and reliably achieved;

' Referring more particularly'to the practice of my invention, Iprovide in a suitable stainless steel production furnace; for" example in an electric 'arc furnace; a meltdown charge of desired raw"'1naterial's, such as stainless steel scrap and/or low-carbon steel scrap and an oxidizing agent such as iron ore, roll scale; or'the' like. I meltdown this charge, therebyiormina bath of ferrous metal covered by an oxidizing slag, and bring the melt to a uniformlyhi'gh temperature. I- add chromium in the furnace either along with the initial meltdown ingredients, using, for example, high carbon ferrochrome or chrome are, or illustratively through the addition' of chrome ore to the bath. and slag, formed indicated above. It usually is advantageous for'such reason as savings inpower consumption, improved furnace operating conditions; and the like, to introduce the chromium additions both along with the initial charge of ingredients and to the bath of metal and slag after meltdown. This is particularly true in using [chrome ore as a source of chromium; for the procedure tends to alleviate crowding; of the furnace and enables the successful consumption of maximum amounts of the ore.

' The supernatant, strongly oxidizing slag formed on the molten metal by virtue of the meltdown, and the high temperatures employed, enable an effective exclusion and/orelimination from the metal bath of any carbon coming from the furnace electrodes and furnace atmosphere, as well as from the raw materials melted. Along with the oxidation of carbon at these high temperatures, there is an incidental oxidation of chromium from the metal; the chromium oxides formed being transferred into the slag.

After achieving a substantially complete meltdown of the ingredients to form a ferrous metal bath of low-carbon content covered by a slag containing oxides of iron and chromium, as indicated above, I charge a suitable reducing agent, such as ferrosilicon into the furnace, for recovering iron and chromium from their oxides in the slag,

thereby increasing efiiciency and economyof production. I prefer to add along with the reducing agent a suitable quantity of basic slag-forming material or materials, such as burnt lime, to assure basic conditions durin the reduction of oxides contained in the slag and a consequent minimum attack by acid slag constituents on the furnace lining.

The ferrosilicon or other suitable reducing agent effects the reduction of oxides of iron and chromium in the slag; thereafter the metallic values resulting from the induction gravitate from the slag into the underlying bath of metal. When the reduction continues to such extent as further recovery of iron and chromium becomes a negligible factor, I draw off the slag and prepare to finish the metal. For this purpose, I build up a finishing slag, illustratively of ferrosilicon and burnt lime, on the metal surface.

With regard to the introduction of nitrogen to melts comprising chromium and iron, I have made the highly important discovery that the presence of soluble metal oxides greatly impairs act reason for impaired solubility of nitrogen in oxide-containing baths is not fully understood by me. It may be that the oxides exist in solution in the form of chromium-iron-oxygen compounds, are inert, and represent a depletion of free chromium which otherwise would be highly active in absorbing nitrogen in the melt; however, I do not wish to be bound by any such explanation.

I deoxidize the metal bath for nitrogen treatment and introduce the nitrogen all preferably during the metal finishing operation and illustratively While the particular finishing slag just .mentioned is on the metal surface. The deoxidizing agent which I employ illustratively is 1.5 pounds of aluminum (as contained in ferro-aluminum) per ton; based on tapping weigh. of the bath. I make the ferro-aluminum addition through the finishing slag by rabbling thoroughly into the melt as furnace conditions permit. This gives a rapid and subs.antially complete deoxidation of any oxides in the melt, leaving the metal clean and ready for treatment with nitrogen.

Either during or after the deoxidation operations, or while the deoxidation is in progress and continuing after the completion of the same, I introduce nitrogen to the melt, advamageously by bubbling a gentle flow of nitrogen gas directly through the molten metal beneath the finishing slag, as by means of an iron pipe or a suitable refractory me.al tube. An efficient absorption of nitrogen by the molten metal rapidly ensues.

' As illustrative of the absorption of nitrogen,

ABSORPTION OF NITROGEN BY MOLTEN STAINLESS STEELS Identification of samples Analysis of Blank Heat No.

MNNHD- 95295050 o n-mom uwmoo Indication of the absorption of nitrogen had by my process for a 12% chromium grade of stainless steel is illustrated by the comparative heats E2783 and E2962. While for a 26% chromium grade it is indicated by the comparative heats E3067 and E3076 and E3077. The amount of nitrogen picked up during the making of any particular heat, of course, is represented by the difference between the values given in Table II for the heat and that given in Table I.

TABLE II Nitrogen absorption data 10 min. Heat 5 lll rm 151mm. 20 gin.

C Cr N E2783 025 12. 24 026 E2962 032 12. 87 E3067 030 031 037 049 E307 179 271 292 E3077 106 148 199 244 In my process, I find advantage in holding the melt at as low a temperature as practical during the deoxidizing and nitrogenizing period, this preferably being below 3000 F. and generally bejlow 2950" F. At temperatures of superheat the deoxidizing ability of aluminum, for example, appears to be less than at relatively low temperatures; and the efficiency of nitrogen recovery increases with decrease in temperature to below temperatures of superheat. The optimum low temperature range, however, varies considerably with many different grades of stainless steel which I produce and thus is determined empirically in specific practice. Examples of the effect of temperature, however, are given in the Table III below:

TABLE III ABSORPTION OF NITROGEN BY MOLTEN In general, in accordance with my practice, I achieve a nitrogen content of approximately 0.07% to 0.30% or higher, by bubbling say 0.6

awaits cubie f-eet of commercial bottled dry nitrogen per minule from aone inch. silica. tube for 5to11i0 minutes? time while holding the: bath at about 2950 F. Commercialbottled. dry'nitrogen isan available and. relatively inexpensive source; of nitrogen gas which lends itself to an ease of operation and precise control.

As an alternative, or in conjunction with the use of commercial bottled nitrogen, I employ any other suitable nitrogen-bearing material or material's desired, as for example ammonia inlatent or nascent stage, cyana-mid'es; -ferro-cyani'de, copper cyanide, urea, or the like. Any additions of the decomposible solid sources of nitrogen conveniently are made through the finishing slag and by rabbling into the underlying metal.

Once the source or sources of nitrogen contact the bath, there begins a rapid, efficient and reliable absorpdve action wherein the nitrogen is absorbed by the chromium-iron alloy metal. Soon, with continued treatment, as with the bubbling gas, the active metal takes up a sufiicient quanity of nitrogen as desired.

Before tapping the heat I add, if desired, amounts of any such supplementary elements as nickel, copper, manganese, cobalt, columbium, titanium, molybdenum, tantalum, tungsten, vanadium or the like, in accordance with standard practice. I then pour the finished melt into suitable molds and allow the poured metal to cool. The resulting product is a clean, nitrogen-containing stainless steel of desired properties.

Thus it will be seen that there is provided in this invention an art and product in which the various objects hereinbefore noted together with many thoroughly practical advantages are successfully achieved. It will be seen that the process of producing stainless steels of a desired nitrogen content may be efficiently and economically attained with a, minimum of effort and/or special equipment.

While in the embodiment of my invention, illustratively set forth above, a bath of ferrous metal containing chromium covered by a slag containing oxides of iron and chromium is prepared by melting down a charge of stainless steel scrap, chrome ore, and mill scale, it will be understood that any desired raw materials such as ordinary scrap, and/or stainless steel scrap as of straight-chromium and chromium-nickel varieties, high carbon ferrochrome and/or chrome ore, and iron oxide and/or nickel oxide may be melted down to form a bath of metal including iron and chromium covered by a supernatant slag comprising metal oxides and these oxides reduced to effect a recovery of the metals, where such procedure is found desirable for the provision of a chromium-rich ferrous metal bath for treatment in accordance with my deoxidizing and nitrogenizing operations.

Likewise, while in the embodiment illustra- Although I. prefer to effectmy deoxidizing; and; nitrogenizing treatment. during and. as a part. of the metal: finishing. stage, either in the furnace, or in the ladle after tapping tht melt from the furnace, or deoxidation in the furnace and nitridi-ng the ladle, good: results. also are: obtained otherwise as by deoxidizing and nitrogenining the metal after. meltdown ofv the charge under oxidizing conditionsand during the ensuing oxide slag reduction period before finishing the: metals As manypossibleembodiments'may bemade of my; invention and. as. many changes may be. made in the embodiments hereinbefore set forth, it will be understood that all matter described herein is to be interpreted as illustrative and not in a, limiting sense.

I claim:

1. In the production in an electric furnace of stainless steel containing at least 0.07 nitrogen wherein there is prepared a bath of stainless steel containing about 10% to 30% chromium and of low carbon content and having oxides of chromium dispersed therethrough, the art which comprises adding to said metalbath a deoxidizing agent of the group consisting of aluminum and calcium-silicon for substantially depleting chromium oxides and thereby increasing the ability of the meal to absorb nitrogen, and adding one of the group consisting of nitrogen gas and a decomposible nitride to said treated bath whereby there is achieved stainless steel of d sired nitrogen content.

2. In the production in an electric furnace of stainless steel containing at least 0.07% nitrogen wherein there is prepared a bath of stainless steel containing about 10% to 30% chromium and of low carbon content and having oxides of chromium dispersed therethrough, the art which comprises dispersing a deoxidizing agent of the group consisting of aluminum and calcium.- silicon through the metal bath for substantially depleting chromium oxides and thereby increasing the ability of the metal to absorb nitrogen, and immediately thereafter introducing nitrogen to the treated bath whereby the desired nitrogen content is achieved.

3. In the production in an electric furnace of stainless steel containing at least 0.07% nitrogen wherein there is prepared a bath of stainless steel containing about 10% to 30% chromium and of low carbon content and having oxides of chromium dispersed therethrough, the art which comprises subjecting said bath to deoxidation treatment with at least one material of the group consisting of aluminum and calcium-silicon for substantially depleting chromium oxides and thereby increasing the ability of the metal to absorb nitrogen, and during said deoxidation treatment bubbling commercial bottled dry nitrogen gas through said bath whereby there is achieved a desired nitrogen content.

4. In the production in an electric furnace of stainless steel containing at least 0.07% nitrogen wherein there is prepared a bath of stainless steel containing about 10% to 30% chromium and of low carbon content and having oxides of chromium dispersed therethrough and having a thin finishing slag thereon comprising lime and ferrosilicon, the art which comprises subjecting the bath to deoxidation treatment with aluminum for substantially depleting chromium oxides and thereby increasing the ability of the metal to absorb nitrogen, and while maintaining the bath at a temperature below 3000 F. introducing 7 nitrogen into said bath whereby the desired quan- Number tities of nitrogen are had. 2,121,056 HARRY TANCZYN. 2,174,740 2,191,790 REFERENCES CITED 5 2,229,140 The following references are of record in the 238L179 file of this patent: UNITED STATES PATENTS Number Name Date 10 1,954,400 Arness Apr. 10, 1934 Number 2,069,205 Arness Feb. 2, 1937 490,835

Name Date Smith June 21, 1938 Graham et a1. Oct. 3, 1939 Franks Feb. 27, 1940 Smith et a1 Jan. 21, 1941 Arness Apr. 28, 1942 Loveless Feb. 27, 1945 Feild June 19, 1945 FOREIGN PATENTS Country Date Great Britain Nov. 16, 1936 

1. IN THE PRODUCTION IN AN ELECTRIC FURNACE OF STAINLESS STEEL CONTAINING AT LEAST 0.07% NITROGEN WHEREIN THERE IS PREPARED A BATH OF STAINLESS STEEL CONTAINING ABOUT 10% TO 30% CHROMIUM AND OF LOW CARBON CONTENT AND HAVING OXIDES OF CHROMIUM DISPERSED THERETHROUGH, THE ART WHICH COMPRISES ADDING TO SAID METAL BATH A DEOXIDIZING AGENT OF THE GROUP CONSISTING OF ALUMINUM AND CALCIUM-SILICON FOR SUBSTANTIALLY DEPLETING CHROMIUM OXIDES AND THEREBY INCREASING THE ABILITY OF THE METAL TO ABSORB NITROGEN, AND ADDING ONE OF THE GROUP CONSISTING OF NITROGEN GAS AND A DECOMPOSIBLE NITRIDE TO SAID TREATED BATH WHEREBY THERE IS ACHIEVED STAINLESS STEEL OF DESIRED NITROGEN CONTENT. 